Prior electric timing circuits of this type have been limited in maximum delay time available from a small energy storage timing capacitor due to a requirement of prior circuits that some current be drawn from the storage/timing capacitor for proper operation of the circuit, thereby discharging the capacitor within a few minutes or a few hours.
Such prior circuits generally require an additional timing capacitor to be charged through a resistor from the energy storage capacitor, or other voltage source, from zero volts up to the threshold voltage of an auxiliary switching device which may be, for example, a four layer diode, a glow lamp, a spark gap, or a zener diode connected in series with a thyristor gate electrode, which conducts as the voltage rises to the threshold level of the particular switching device and connects the remaining voltage of the energy storage capacitor to an electric fuze element.
Other prior circuits utilize a small current from the storage capacitor through a limiting resistor in series with a small coulometer device which electrolytically plates a limited quantity of metal from one electrode to the other until the available electrode metal is depleted, constituting the end of the timing cycle, at which time the electrical resistance of the coulometer device increases resulting in an increased voltage across the device which is used to trigger a thyristor or transistor, connecting the remaining energy storage capacitor voltage to an electric detonator.
The prior circuits which utilize an additional timing capacitor or auxiliary timing device not only require current from the storage capacitor with resulting limits on maximum delay time, but also have the disadvantage that extraordinary or unforeseen leakage currents, particularly under high ambient temperature conditions, may discharge the energy storage capacitor prior to the end of the timing cycle, as determined by the auxiliary timing components, and result in a dud, or unexploded shell, bomb, or mine with possible subsequent danger to friendly personnel.
Other timing circuits, for example U.S. Pat. No. 3,700,968, utilize a decreasing-voltage threshold detector to connect the remaining voltage on the energy storage capacitor, when the capacitor voltage decreases to the threshold value, to the electric fuze element without suffering the disadvantage of a possible dud; however, that circuit requires current from the capacitor in order to maintain the threshold switch in the non-conducting state until the voltage threshold is reached, and consequently reduces the maximum delay time. That circuit also requires a relatively expensive current limiting resistor in the order of 1000 megohms or higher for its maximum delay time.